1. Field of the Invention
The present invention relates to an apparatus and method for performing a tracking control in a system for reading or writing data from/on a storage medium by means of a light beam.
2. Description of the Related Art
FIG. 15 is a block diagram showing a conventional tracking controller for use in a system for reading or writing data from/on a storage medium by means of a light beam (e.g., an optical disc drive). In the example illustrated in FIG. 15, the storage medium (typically in a disk shape) 1 that has been loaded into this system is a DVD-R disc. As shown in FIG. 15, the optical head 2 includes a semiconductor laser diode 2a (which emits a laser beam with a wavelength of about 650 nm, for example), a collimator lens 2b, a polarization beam splitter 2c, a wave plate 2d, a convergent lens 2e, a photo-detecting hologram 2f, a detector lens 2g, a plus-first-order photodetector 2h, a minus-first-order photodetector 2i and a tracking coil 2j. 
The light beam that has been emitted from the semiconductor laser diode 2a is collimated by the collimator lens 2b into a parallel beam, which is passed through the polarization beam splitter 2c and the wave plate 2d and then focused by the convergent lens 2e onto the data storage layer of the disc 1.
The focused beam is reflected back from the data storage layer and then passes the same convergent lens 2e and wave plate 2d again following the same optical path. Thereafter, the reflected light beam is split by the polarization beam splitter 2c into two, one of which is separated from the original optical path so as to be incident onto the photo-detecting hologram 2f. On receiving the separated light beam, the photo-detecting hologram 2f diffracts the light beam by splitting it into a minus-first-order light beam for detecting tracking errors (which will be referred to herein as a “tracking-error-detecting minus-first-order light beam”) and a plus-first-order light beam for detecting focus errors (which will be referred to herein as a “focus-error-detecting plus-first-order light beam”). Subsequently, the tracking-error-detecting minus-first-order light beam and the focus-error-detecting plus-first-order light beam are incident onto the minus-first-order photodetector 2i and the plus-first-order photodetector 2h, respectively, and then converted into electric signals.
The output of the plus-first-order photodetector 2h is a focus error signal representing the focusing state of the light beam on the data storage layer of the disc 1. The position of the convergent lens 2e is controlled such that the focus error signal equals zero. This type of control is called a “focus control”. The focus control is well known in the art, and the detailed description thereof will be omitted herein.
The minus-first-order photodetector 2i includes two divided detecting areas A and B as shown in FIG. 16. The direction in which the boundary between these two detecting areas A and B extends corresponds with the direction in which the tracking-error-detecting minus-first-order light beam follows the tracks on the disc (i.e., the tracking direction). That is to say, a light beam that has been reflected from an outside portion of a track enters the detecting area A, while a light beam that has been reflected from an inside portion of the track enters the detecting area B. Then, the tracking-error-detecting minus-first-order light beams that have been detected at the detecting areas A and B are converted into electric signals, which are output as signals TR1 and TR2 to a tracking error detecting section 3.
The tracking error detecting section 3 generates a tracking error signal from the output signals TR1 and TR2 of the minus-first-order photodetector 2i by a push-pull method. Also, the tracking error detecting section 3 normalizes the tracking error, detected by the push-pull method, with (TR1+TR2), i.e., the sum of the output signals of the minus-first-order photodetector 2i. Accordingly, the TE signal, which is the output signal of the tracking error detecting section 3, is given by the following Equation (1):TE=(TR1−TR2)/(TR1+TR2)  (1)
In Equation (1), the tracking error, detected by the push-pull method, is divided by the sum (TR1+TR2) of the signals TR1 and TR2. This is done to prevent the tracking error detection sensitivity from being affected by any change in the intensity of the light beam that has been reflected from the disc 1 even if the output power of the semiconductor laser diode 2a or the reflectance of the disc 1 changes while a read or write operation is being carried out on the disc 1.
Next, a tracking control section 4 subjects the TE signal to phase compensation, transforms its signal intensity into a variation in current, and supplies the current to the tracking coil 2j. In accordance with the output of the tracking control section 4, the tracking coil 2j drives the convergent lens 2e in the disc radial direction. In this manner, the light beam can be controlled so as not to lose tracks.
In accordance with a track jump instruction from a microcomputer 10, a transport stage 9 controls a move of the optical head 2 in the disc radial direction if the move is too big for the tracking coil 2j to cope with. For example, to move the optical head 2 to right under a predetermined track, the optical head 2 is roughly moved first by the transport stage 9 to around the predetermined track and then precisely controlled by the tracking coil 2j so as to catch and keep that track.
Japanese Laid-Open Publication No. 60-138740 discloses a tracking controller including a voltage generator and a multiplier. The voltage generator generates a voltage to be multiplied by a tracking error (TE) signal. The multiplier multiplies the TE signal by the output voltage of the voltage generator. In this tracking controller, based on the level of the TE signal when the light beam crosses a track on the disc 1 while the tracking servo loop is OFF, the voltage generator defines the output voltage such that the level of the TE signal, which is the output of the multiplier, equals a predetermined value.
In the conventional tracking controller, the amplitude of the TE signal, detected by the Equation (1) described above, on an information recorded area of the disc 1 is supposed to be equal to, or at least almost equal to, that of the TE signal on an information unrecorded area thereof.
FIGS. 17A and 17B show the amplitudes of the TE signals on a data unrecorded area and on a data recorded area of a DVD-R, respectively. As shown in FIGS. 17A and 17B, the sum of the signals TR1 and TR2 is greater in the unrecorded area than in the recorded area. Accordingly, the TE signal in the recorded area has the greater amplitude than the TE signal in the unrecorded area.
Consequently, if recorded areas and unrecorded areas are both present on the same disc 1, then the TE signal changes its amplitude location by location (i.e., depending on whether the light beam spot is located on a recorded area or on an unrecorded area of the disc 1). Accordingly, the gain of the tracking control loop for the data recorded area is also different from that of the tracking control loop for the data unrecorded area. As a result, the accuracy or stability of the tracking control decreases significantly.